The names of the provinces of Ireland are, actually, the names of old tribes. So their nominative form is actually a nominative plural form, and their genitive is a genitive plural:Connachta (= the people of Connaught) > ConnachtMumha (=The people of Munster) > MumhanLaighin (=The people of Leinster) > LaigheanUlaidh (= The people of Ulster) > Uladh.

Also, in the dative case, they may have an -ibh ending because they are dative plurals: in Ultaibh, go Laighnibh...

The names of the provinces of Ireland are, actually, the names of old tribes. So their nominative form is actually a nominative plural form, and their genitive is a genitive plural:Connachta (= the people of Connaught) > ConnachtMumha (=The people of Munster) > MumhanLaighin (=The people of Leinster) > LaigheanUlaidh (= The people of Ulster) > Uladh.

Also, in the dative case, they may have an -ibh ending because they are dative plurals: in Ultaibh, go Laighnibh...